Largest Meta-Analysis

Flegal et al (2013) conducted a meta-analysis of over 100
previously published research papers connecting body weight and
mortality risk among 2.88 million study participants living around
the world. Using standardized BMI
categories (WHO 1997) relative to those classified as normal
weight:

Those classified with Grade 1 obesity did not have higher
a mortality overall

Those with Grades 2 and 3 obesity were associated with significantly
higher all-cause mortality

Possible explanations of these finding include:

Overweight and obese people may get better medical care.

Heavier people may have better survival during a medical
emergencies such as infections or surgery.

Reverse causation, implying being sick can make you thin.

Studies showing contrary results used a variety of different
BMI categories with different cut-points for the various weight
groups.

Overweight people in theses studies who tend to live longest
may not be fatter than most people at all, but in fact, average
weight, since 33% of U.S. adults are clinically overweight and
an additional 36% are obese.